A typical MEMS device is usually built on a sacrificial layer, such as organic material, SiO2, diamond like carbon (DLC), or Si. The sacrificial layer is removed after processing of the MEMS structure is complete. This step in the process is typically referred to as the release process, or step. Depending on the sacrificial material used, the release process can either be a wet process, such as HF etching of SiO2, or dry processes such as RIE of an organic material, or XeF2 if Si is used.
MEMS structures built using multiple layers often bend after release due to a bending moment caused by the different built-in stresses of the thin films. This unintentional and undesired curvature, or bending, of a MEMS device not only changes the driving characteristics of the MEMS derives, such as flatness of micro-mirrors, pull-in voltage of actuators but may also impose a serious reliability problem. With the help of mechanical modeling, a multilayer beam MEMS device may be designed, or built up, such that the bending torques are effectively cancelled with the selection of layer thickness and stress magnitudes. For a single layer device (gyroscopes, accelerometers, resonators and the like), such an approach is typically not desired or feasible, and any stress gradients existing in the material must be carefully controlled. It has been found that a single layer SiO2 cantilever beam fabricated with single-crystal Si used as sacrificial layer exhibits a curvature and bends down after the beam is released. The magnitude of the bending observed cannot be explained by the stress gradients typically occurring in a PECVD SiO2 layer only. Even a well balanced multilayer beam could exhibit uncontrolled deformation when it uses Si as a sacrificial layer.